It is generally known to use Coriolis effect mass flow meters to measure mass flow and other information for materials flowing through a conduit in the flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit or conduits, and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flow meter, a driving force applied to the conduit causes all points along the conduit to oscillate with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit to have a different phase. For example, the phase at the inlet end of the flow meter lags the phase at the centralized driver position, while the phase at the outlet leads the driver. Pickoff sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pickoff sensors are processed to determine the phase difference between the pickoff sensors. The phase difference between the two or more pickoff sensors is proportional to the mass flow rate of material through the conduit(s).
Meter electronics generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pickoff sensors. The driver may comprise one of many well known arrangements, however, a magnet mounted to one conduit and an opposing drive coil mounted to the other conduit or a fixed base has received great success in the flow meter industry. An alternating current is passed to the drive coil for vibrating both conduits at a desired flow tube amplitude and frequency. It is also known in the art to provide the pickoff sensors as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current which induces a motion, the pickoff sensors can use the motion provided by the driver to induce a voltage. The general operating principles of the driver and pickoff sensors are generally known in the art.
In many applications, the drive signal is determined at least in part by the pickoff signals. Once an initial drive signal is applied to the driver to induce the meter to vibrate, the pickoff sensors maintain a certain velocity. It is known in the art for the meter electronics to control the drive signal according to “displacement” or “positional” type control. In other words, meter electronics generates and applies drive signals to the driver that maintain a certain amplitude in the pickoff signals. The amplitude of the voltage signal generated by the movement of the pickoff is generally designed to be proportional to the amplitude of the sinusoidal displacement of the flow tube, and this relationship is often expressed in terms of voltage/hertz. The pickoff signal amplitude is a function of the following components: the frequency of pickoff oscillation, the pickoff's magnetic field, the pickoff coil wire length, and the pickoff's amplitude of motion. It is typical in the art for the system frequency of operation to be measured by the electronics, while both the magnetic field and the coil wire length are assumed to be constants. Control of the flow tube displacement amplitude is therefore accomplished by maintaining a target pickoff signal voltage amplitude, often expressed in terms of voltage/hertz.
A problem arises with this amplitude relationship when the flow meter is exposed to either high environment temperatures or high process fluid temperatures. Both can cause the driver and pickoff sensor temperatures to increase. As the temperature increases, the magnetic field of both the driver and the pickoff sensors decreases thus causing the pickoff output voltage to decrease If the drive signal is set to maintain the pickoff output voltage as a constant voltage/Hertz relationship, the drive signal is then increased, which increases the flow tube displacement amplitude in order to increase the pickoff voltage amplitude. The resulting higher flow tube amplitude causes several unwanted effects. From a structural viewpoint, the higher tube amplitude results in higher tube stresses. Differing flow tube amplitudes can also drive unwanted vibrational modes which impress themselves onto the pickoff signal causing measurement errors. Additionally, the higher tube amplitude requires more drive power. In many situations, the power available may be limited by safety approval ratings or by manufacturing specifications. The drive power requirements are increased to an even greater degree in high temperature environments because the drive magnet's efficiency decreases. There is a need in the art to provide a method for maintaining a constant flow tube amplitude in the presence of high temperatures. The present invention solves this and other problems and an advance in the art is achieved.